This invention relates to systems and methods for ion implantation of semiconductor wafers and other workpieces and, more particularly, to methods and apparatus for adjusting the profile of a scanned ion beam.
Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. An ion implanter which utilizes a combination of beam scanning and target movement is disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
In the beam scanning approach, an ion beam is deflected by a scanning system to produce ion trajectories which diverge from a point, referred to as the scan origin. The scanned beam then is passed through an ion optical element which performs focusing. The ion optical element converts the diverging ion trajectories to parallel ion trajectories for delivery to the semiconductor wafer. Focusing can be performed with an angle corrector magnet or with an electrostatic lens.
The scanning system typically comprises scan plates to deflect the ion beam and a scan generator for applying scan voltages to the scan plates. The voltages on the scan plates produce an electric field in the region between the scan plates that deflects ions in the ion beam. A scan voltage waveform is typically a sawtooth, or triangular, waveform, which, in combination with wafer movement, distributes the ion beam over the wafer surface.
Uniform implantation of ions across the surface of the semiconductor wafer is an important requirement in many applications. The uniformity may be expressed as a percentage variation in ion dose over the wafer surface. In one approach, a uniformity optimization process is employed to achieve a desired uniformity over the wafer surface. In prior art ion implantation systems, a linear scan waveform is initially applied to the scan plates, so that the scan plates sweep the ion beam in one dimension at a constant rate. The uniformity of the scanned ion beam is measured, and the scan waveform is adjusted to cause a change in the ion beam distribution across the semiconductor wafer. The scan waveform is typically piece-wise linear. Adjustment of the scan waveform involves adjusting values which define the slopes of each of the piece-wise linear segments of the scan waveform. In general, the initial linear scan waveform may not produce the desired uniformity across the semiconductor wafer, and an adjusted scan waveform may be required. The measurement of beam uniformity and adjustment of the scan waveform are repeated until the desired uniformity is achieved.
A typical user of the ion implantation system may need to set up multiple implants of different ion species at different energies and doses. The setup process is typically repeated for each set of implant parameters. The setup process is typically time consuming and reduces the throughput of the ion implanters.
In some cases, the setup process for ion implanters is automated. The automated process may permit a predetermined number of iterations of the uniformity optimization process, wherein the beam uniformity is measured, the scan waveform is adjusted and the beam uniformity is again measured. If the desired uniformity is not achieved in the predetermined number of iterations, the optimization process is terminated. Accordingly, a parameter known as success rate is associated with the automated uniformity optimization process. The process is considered a success if the desired uniformity is achieved within the predetermined number of iterations. In practice, even the automated optimization process can be time consuming and can reduce ion implanter throughput.
Accordingly, there is a need for improved methods and apparatus for optimizing the uniformity of a scanned ion beam.
According to a first aspect of the invention, a method is provided for adjusting the profile of a scanned ion beam. The method comprises the steps of (a) measuring a spatial distribution U(x) of the unscanned ion beam in a scan direction x, (b) scanning the ion beam at an initial scan speed W0(x) to produce a scanned ion beam, (c) measuring a beam profile S(x) of the scanned ion beam, (d) determining if the measured beam profile is within specification, (e) if the measured beam profile is not within specification, determining a scan speed correction C(x) that produces a desired profile correction, using a calculation which is based on the spatial distribution U(x) of the unscanned ion beam, and (f) scanning the ion beam at a corrected scan speed Wc(x), which is based on the initial speed W0(x) and the scan speed correction C(x), to produce a corrected beam profile. In a preferred embodiment, the corrected beam profile is uniform within preselected limits.
The scan speed correction may be determined by selecting a candidate scan speed correction, convolving the candidate scan speed correction with the spatial distribution U(x) of the unscanned ion beam to produce a result and determining if the result is sufficiently close to the desired profile correction. The candidate scan speed correction may be selected by performing a search for the scan speed correction C(x) that produces the desired profile correction. The search may comprise a multi-dimensional search algorithm, such as a downhill simplex algorithm. In a preferred embodiment, the desired profile correction comprises 1/S(x) and the corrected scan speed Wc(x) comprises W0(x)/C(x).
Steps (c)-(f) may be repeated until the beam profile is within specification. The adjustment process may be terminated after a predefined number of iterations of steps (c)-(f) if the beam profile is not within specification and is not improving on successive iterations.
According to another aspect of the invention, apparatus is provided for adjusting the profile of a scanned ion beam. The apparatus comprises means for measuring a spatial distribution U(x) of the unscanned ion beam in a scan direction x, means for scanning the ion beam at an initial scan speed W0(x) to produce a scanned ion beam, means for measuring a beam profile S(x) of the scanned ion beam, means for determining if the measured beam profile is within specification, means for determining a scan speed correction C(x) that produces a profile correction, if the measured beam profile is not within specification, using a calculation which is based on the spatial distribution U(x) of the unscanned ion beam, and means for scanning the ion beam at a corrected scan speed Wc(x), which is based on the initial scan speed W0(x) and the scan speed correction C(x), to produce a corrected beam profile.
According to a further aspect of the invention, a method is provided for estimating a beam profile of an ion beam that is scanned in a scan direction x at a scan speed W(x). The method comprises the steps of measuring a spatial distribution U(x) of the unscanned ion beam, and calculating a beam profile S(x) based on the spatial distribution U(x) of the unscanned ion beam and the scan speed W(x). The beam profile S(x) may be calculated by convolving the spatial distribution U(x) and the scan speed W(x).